"Video-on-demand" (VOD) viewing is a method for allowing consumers to select and view an arbitrary video file such as a motion picture at any time from a large number of choices supplied by a service provider. One of the many factors critical to the success of such a VOD consumer service is "latency", e.g., the time which passes from the moment the customers make the initial title selection to when the video is actually available for viewing. Generally it has been found that nominally a delay on the order of 60 seconds from time of selection to actual commencement of display of the image is acceptable. Yet another critical factor in the success of such services is whether the service provider can offer a large quantity and variety of choices in content to their customers.
In accordance with present day techniques, on-demand display of motion picture data has generally been effected by storage of the associated digitized video files on magnetic disks. The storage requirements for such an approach however, can become enormous. A typical full length motion picture may average perhaps 110 minutes of video data. Assuming a representative conventional and well-known video compression format such as MPEG2 at a nominal data rate of 3 megabits per second, even a single motion picture will require approximately 2 gigabytes of disk storage. Thus, it is readily apparent that storage of large numbers of motion picture data files on disk may not be cost effective but nevertheless necessary in order to satisfy the first mentioned consumer criteria of a low latency access time on the order of seconds. Unfortunately then, one of the major cost components of a commercially acceptable VOD server is simply the disk storage required to store the aforementioned MPEG2 or other data files for all of the selections that the service provider wishes to offer.
Illustrated schematically in FIG. 1A is one simplified approach to VOD systems. In this system, a plurality of disk storage devices 24 are provided having stored thereupon the desired content which, in the case under consideration, is a variety of desired full-length digitized motion pictures. In response to a user selection of a particular desired title, the file is read off the disk storage 24 into an appropriate video server 2, which then delivers the image of the film on the display 3 to the end user. This system successfully addresses the aforementioned latency problem inasmuch as the disk storage has a relatively fast access time, e.g., it is sufficiently responsive to the user's selection so as to mount and begin displaying the desired selection after perhaps one or two seconds after receipt of the user's choice. However, as pointed out, disk storage, although having the advantage of fast access time, unfortunately also suffers from the concomitant attribute of being quite expensive relative to other known mass storage devices. Thus such systems have enjoyed little success.
In recognition of this fact, illustrated in FIG. 1B is yet another schematic of an alternative VOD system which addresses the drawbacks of the system of FIG. 1A. Essentially the technique employed as illustrated in FIG. 1B for reducing the cost of on-line disk storage of the video data is to simply use a less expensive storage media such as magnetic tape storage 4. Thus, a plurality of magnetic tape carrousels may be provided, having associated therewith a corresponding plurality of video titles stored on such magnetic tape. In response to user input specifying a desired title to be viewed, the carousel operates to mount the particular tape containing the desired video image file on an appropriate tape drive 5. The tape drive then reads the tape and generates a video data stream viewable at the appropriate display device 3.
A characteristic of such magnetic tape storage and associated carrousels and players is that they are substantially less expensive than the disk storage and drive approach shown in FIG. 1A. However, as should be readily apparent given the nature of tape drives, using such a tape media to store the video data increases the undesirable latency between the user selection and the availability of the motion picture for viewing. such delay is not always an acceptable alternative, particularly in a commercial setting. This increased latency in the magnetic tape approach is due to the time required for an automated tape library to load the tape from the carousel, to locate the beginning of the video data file, and to move the data from tape to disk 1 if playback will not be directly from the tape drive 5.
On the latter point, in addition to the possibility of moving the entire data file from tape 4 to disk 1 prior to viewing (if the tape data rate exceeds that of the viewing data rate), such prior systems alternatively provided for viewing the video data directly from the tape storage. In either event, however, the aforementioned latency problem was nevertheless still present, as well as the need for providing dedicated tape drive units for the duration of the viewing time for each viewer.
Thus, in summary, the problem of providing demand video viewing from digitized motion picture has been solved by 3 basic approaches. First, all such data in some systems has been stored entirely on magnetic disk, giving rise to prohibitive cost constraints. Secondly, still other systems have at least saved some cost in magnetic disk by providing selected and less frequently requested movie selections on tape. However, this requires the viewer to wait for video data to be restored from tape before viewing, thereby contributing to the undesirable latency problem previously described. Finally, still other systems have provided for cost savings by storing movie data on tape and further providing for viewing of the movie data directly from such tape. This proposed solution has also been proven to not be cost effective inasmuch as it requires a tape drive to be available for each end user or viewer.